In an electrically powered steering apparatus used as a steering system for an automobile, for example, a steering torque sensor commonly senses the steering torque applied to a steering shaft from a steering wheel by the steering operation of the driver. The steering torque sensor is normally configured from a magnetostrictive torque sensor. The steering shaft functions as a rotating shaft rotated by the rotational force from a steering operation. The steering shaft constitutes the rotating shaft in the steering torque sensor. The electrically powered steering apparatus controls the driving of a steering force auxiliary motor according to a torque signal detected from the steering torque sensor, and reduces the steering force for the driver to provide a pleasant steering feel.
As described above, magnetostrictive torque sensors are well known as steering torque sensors used in electrically powered steering apparatuses. In such a magnetostrictive torque sensor, magnetostrictive films 102A, 102B that have inverse magnetic anisotropy 103, 104 with respect to each other are formed at two specific upper and lower locations on, e. g., the surface of the steering shaft (also referred to as the rotational shaft) 101, as shown in FIG. 12. The magnetostrictive torque sensor 100 has a configuration in which a non-contact system is used in which sensor coils 106A, 106B sense changes in the magnetostrictive characteristics of the magnetostrictive films 102A, 102B that correspond to the torsion of the steering shaft 101. The changes are sensed when an input torque is applied to the steering shaft 101 from the steering wheel, as shown by the arrow 105.
FIG. 13 shows the principle of input torque sensing in the sensor configuration of the magnetostrictive torque sensor 100. The characteristic VT1 represents an input torque/output characteristic obtained based on an output signal from the sensor coil 106A. The characteristic VT2 represents an input torque/output characteristic obtained based on an output signal from the sensor coil 106B. The slopes of the characteristics VT1 and VT2 are opposite each other because the directions of magnetic anisotropy 103, 104 of the magnetostrictive films 102A, 102B are opposite. The characteristic VT3 represents an input torque/output characteristic created by using the characteristics VT1 and VT2 and finding the difference between the two. The input torque applied to the steering shaft can be determined based on the characteristic VT3. In practice, point B of the characteristic VT3 is set as the origin, the part to the right is set as the positive region, and the part to the left is set as the negative region. Information about the rotating direction and extent of the input torque can be obtained from this characteristic VT3.
In the process for manufacturing the magnetostrictive torque sensor 100, it is necessary to perform a step in which magnetostrictive films 102A, 102B (in a wider sense, magnetostrictive regions) are formed over a specific surface of part of the steering shaft 101; i.e., over the entire circumferential surface of a specific axial width in the columnar rotating shaft 101, and these magnetostrictive films are then provided with magnetic anisotropy 103, 104. Conventional methods for providing magnetostrictive films with magnetic anisotropy in the manufacture of a magnetostrictive torque sensor 100 involve applying a twisting torque to a rotating shaft on which magnetostrictive platings (magnetostrictive films) are formed, e.g., by an electroplating process, thus creating stress in the circumferential surface of the rotating shaft. This is followed by heat treating the rotating shaft in a thermostat while the shaft is kept under stress (see Japanese Laid-open Patent Application No. 2002-82000, for example).
Magnetostrictive torque sensors have required that the inverse magnetostrictive characteristics created by magnetic anisotropy in the magnetostrictive films formed on the rotating shaft be maintained over a long period of time. However, with conventional magnetostrictive torque sensors, the magnetostrictive films have typically been formed over the entire circumferential surface of the columnar rotating shaft by electroplating. Therefore, moisture sometimes adheres to the plated parts, which are the magnetostrictive films, causing corrosion or peeling, or the magnetostrictive films are corroded or caused to peel by external causes, thereby causing the magnetostrictive films to fail.
In a conventional magnetostrictive torque sensor, it has been impossible to reliably detect failures in the magnetostrictive films. The reason for this is because when a change occurs in sensor output, it has been impossible to determine whether the change is based on a change in the surrounding temperature, a change in the input torque, or a failure in the magnetostrictive films.
Since it has been impossible to detect failures in the magnetostrictive films of a conventional magnetostrictive torque sensor 100 as described above, one idea has been to use a sensor configuration such as is shown in FIG. 14, for example, to deal with failures in cases in which failures are assumed to have occurred. The sensor configuration shown in FIG. 14 is obtained by adding an identically structured magnetostrictive film configuration 107A to the rotating shaft 101 in a magnetostrictive film configuration 107 composed of the above-described magnetostrictive films 102A, 102B. Specifically, two sets of magnetostrictive films 102A, 102B are formed at intervals on the rotating shaft 101. According to this configuration, in cases in which the top set of magnetostrictive films has failed, the failure can be reliably detected because the sensor output value from the top set of magnetostrictive films differs from the sensor output value of the bottom set of magnetostrictive films. The idea is that failures in the magnetostrictive films can be detected by providing this double set of magnetostrictive films 102A, 102B.
However, if the sensor configuration shown in FIG. 14 is used, problems are raised in that an extremely long space is needed to provide the magnetostrictive films in the rotating shaft 101. The entire length L2 must currently be 96 mm because the magnetostrictive film width W1 must be 18 mm and the interval between the magnetostrictive films L1 must be 8 mm in order to ensure the ensure the required detection performance of the magnetostrictive films and to eliminate the effects that adjacent magnetostrictive films have on each other.
Recently, the width of the rotating shaft in the axial direction in an electrically powered steering apparatus for an automobile is currently limited to about 100 mm because of design considerations when a magnetostrictive film is formed by plating. It has also sometimes been difficult to ensure a width of 100 mm, depending on the type of vehicle. Consequently, in cases in which the axial film width is 96 mm as described above, it is difficult to use the magnetostrictive torque sensor in an electrically powered steering apparatus in practical terms.
Thus, a need has existed for minimizing the axial length of a steering shaft on which magnetostrictive films can be formed in order to be able to use a magnetostrictive torque sensor in many different types of vehicles.
Therefore, a need has existed for a magnetostrictive torque sensor, and also an electrically powered steering apparatus that uses this sensor, that has a simple configuration wherein it is possible to easily and reliably detect failures in the magnetostrictive films that sense an input torque and are formed on the rotating shaft, and wherein the space for forming the magnetostrictive films on the rotating shaft can be minimized.